Resolving the Issue of "The Expression is Unused" in Kotest with IntelliJ

Описание: Discover the solution to the "The expression is unused" warning in Kotest while using IntelliJ and learn how to properly structure your tests.
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This video is based on the question https://stackoverflow.com/q/65577709/ asked by the user 'Thomas Martin' ( https://stackoverflow.com/u/7346454/ ) and on the answer https://stackoverflow.com/a/65595916/ provided by the user 'sksamuel' ( https://stackoverflow.com/u/2048448/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding and Fixing the "The Expression is Unused" Warning in Kotest

When working with Kotlin and the Kotest framework in IntelliJ, you might encounter an annoying warning that states, "The expression is unused." This can happen when your test cases are not set up properly, resulting in the IDE's syntax highlighting failing and even preventing the tests from running. In this guide, we'll explore the problem and guide you through the solution step by step.

The Problem

Consider the following Kotlin test written with Kotest:

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In this example, you may see the warning "The expression is unused," and syntax coloration may not work properly. Additionally, you might find that the test is not found when you try to run it.

Understanding the Source of the Issue

The warning and syntax issues arise because the structure of the test does not comply with how Kotlin's DSL (Domain Specific Language) works in Kotest. To appreciate the solution, we must first understand how we can build a test from scratch while utilizing some Kotlin features effectively.

Using Kotlin's Extension Functions

In Kotlin, we can create extension functions that allow us to add new functionalities to classes we don’t control. For example, we can define a function:

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This function can be invoked on any string as follows:

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Leveraging Lambda Function Syntax

Kotlin allows for lambda arguments to be placed outside of the parentheses. This means we can simplify the syntax to:

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Making Functions Infix

We can also mark functions as infix, eliminating the need for the dot operator when calling them. The test function can be redefined as:

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This leads to the following concise syntax:

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Utilizing Operator Functions

To take it a step further, any function named invoke and defined as an operator can be called without explicitly mentioning its name. Thus, we can redefine our test to:

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And our final test will look like:

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If you prefer, you can structure the test slightly differently by placing the braces on a new line for clarity:

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Conclusion

In summary, the warning "The expression is unused" in your Kotest tests is a result of improper structure in your Kotlin DSL usage. By leveraging Kotlin's extension functions, lambda syntax, infix functions, and operator functions, you can resolve this issue effectively. Make sure your test case is structured properly for IntelliJ to recognize and execute the tests without warnings.

By following these steps, you can ensure that your Kotest tests not only compile correctly but also maintain clean and readable syntax. Happy testing!